Fluorescent lamps are low-pressure mercury arc discharge devices which have electrodes at each end of an elongated glass envelope and which contain a phosphor coating on the inner surface of the glass envelope. Such lamps experience a gradual decrease in light output with increasing hours of use. A variety of factors contribute to the drop-off in light output during lamp operation. These factors include deposits of impurities from the cathode; the formation of various mercury compounds due to the bombardment of the phosphor by atoms and ions of mercury; changes in the phosphor itself; and changes in the glass envelope, particularly where it may be subject to ultraviolet radiation. The ability of such lamps to resist the drop-off in light output is generally termed lumen maintenance which is measured as the ratio of light output at a given life span compared to an initial light output and expressed as a percentage.
While the decrease in light output with time is an occurrence for all fluorescent lamps, it presents a greater problem for high output and very high output lamps than for normally loaded lamps.
The art and artisians of lamp design have expanded much time and money in an effort to solve these problems. Although the problem of lumen maintenance still exists, it has been alleviated to some extent. Some of the solutions proposed involve changing the inner surface chemistry or the composition of the glass tubing used for the lamp envelope.
U.S. Pat. No. 4,228,206, which issued to Fabisak on Oct. 14, 1980, discloses a method of changing the surface chemistry of the inner surface of the glass tubing by reducing the alkali content thereon. The method involves processing the glass tubing by reacting an acidic gas with the softened glass surface of the tubing as it is being drawn from a glass melt. However, because of the corrosive nature of the reacting gas, the components of the apparatus employed must be acid-resistant.
European Patent Application Nos. 0146187 and 0146188 disclose various glass compositions which resist both phosphor poisoning and mercury penetration. The phosphor poisoning is controlled by restricting the mobility of sodium ions or by incorporating little or no Na.sub.2 O in the glass. Mercury penetration is controlled by the presence of a large quantity of alkaline earth metal oxides in the glass. Although the glass compositions disclosed provide satisfactory results, the glass compositions are more expensive than standard soda lime glass. Also, manufacture of the glass compositions taught requires changing the ingredients in the glass melt.